Investigation of SARS-CoV-2 seroprevalence in relation to natural infection and vaccination between October 2020 and September 2021 in the Czech Republic: a prospective national cohort study.

Objective: Examine changes in SARS-CoV-2 seropositivity before and during the national vaccination campaign in the Czech Republic.

Design: Prospective national population-based cohort study.

Setting: Masaryk University, RECETOX, Brno.

Participants: 22 130 persons provided blood samples at two time points approximately 5-7 months apart, between October 2020 and March 2021 (phase I, before vaccination), and between April and September 2021 (during vaccination campaign).

Outcome Measures: Antigen-specific humoral immune response was analysed by detection of IgG antibodies against the SARS-CoV-2 spike protein by commercial chemiluminescent immunoassays. Participants completed a questionnaire that included personal information, anthropometric data, self-reported results of previous RT-PCR tests (if performed), history of symptoms compatible with COVID-19 and records of COVID-19 vaccination. Seroprevalence was compared between calendar periods, previous RT-PCR results, vaccination and other individual characteristics.

Results: Before vaccination (phase I), seroprevalence increased from 15% in October 2020 to 56% in March 2021. By the end of phase II, in September 2021, prevalence increased to 91%; the highest seroprevalence was seen among vaccinated persons with and without previous SARS-CoV-2 infection (99.7% and 97.2%, respectively), while the lowest seroprevalence was found among unvaccinated persons with no signs of disease (26%). Vaccination rates were lower in persons who were seropositive in phase I but increased with age and body mass index. Only 9% of unvaccinated subjects who were seropositive in phase I became seronegative by phase II.

Conclusions: The rapid increase in seropositivity during the second wave of the COVID-19 epidemic (covered by phase I of this study) was followed by a similarly steep rise in seroprevalence during the national vaccination campaign, reaching seropositivity rates of over 97% among vaccinated persons.